Optical wavelength interferometry has long been recognized as a method for precise length determination. For example, interferometrically controlled machine tools have been utilized in highly specialized manufacturing operations, including production of difraction gratings, for several decades. In these cases, the scale of the distances measured was of the order of the fringes produced by the interference between the reference and signal light beams, i.e. for visible light, the scale was of the order of one hundred nanometers. Such measurements have been exclusively laboratory measurements, however, with great care taken to provide environmental control against unwanted vibration, as well as changes in the ambient temperature, humidity and related factors.
With the advent of the laser and its greater coherence length, the qualitative aspects of the interferometry measurement technique did not change. Now technical problems arose, however, associated with maintaining equal path lengths in the interferometer beam channels or arms. The use of a laser interferometer to measure time-varying displacements has been widely recognized, and descriptions of laboratory type laser interferometers have been published by S. M. Khana et al, 44 J. Acoust. Soc. Am. 1555(1968); P. R. Dragsten et al. 60 J. Acoust. Soc. Am. 665(1976); and R. M. De LaRue et al, 119 Proc. IEE (1972).